For several years, fiber optic sensors, and in particular DTS systems, have provided higher bandwidth, inherently safe operation (no generation of electric sparks), and immunity from EMI (Electromagnetic Interference) for parameter measurements.
For example, the temperature profile parameter and other parameter profiles along the fiber can be monitored. The resulting distributed measurement is equivalent to deploying a plurality of conventional point sensors, which would require more equipment and increase operational costs. Each conventional electrical point sensor would require multiple electrical leads and this would add to a large and expensive cable bundle as the number of point sensors increase.
When an optical fiber is excited with a laser light having a center wavelength λ, most of the light is transmitted. However, small portions of incident light λ and other excited components are scattered backward and forward along the fiber. The amplitude of the other excited components depends on the intensity of the light at center wavelength λ and the properties of the optical fiber. In the measurement of distributed temperature using Raman scattering, three components are of particular interest. The three components are Rayleigh back-scattered light, which will have a similar wavelength λ as the original laser wavelength, Raman Stokes and Raman anti-Stokes components which have longer and shorter wavelengths than the original wavelength λ. These three components can be separated by optical filters and received by photo detectors to convert light to electrical signals. A ratio between the temperature sensitive Raman anti-Stokes intensity to the temperature insensitive Rayleigh or largely temperature insensitive Raman Stokes intensity forms the basis of a Raman based distributed temperature measurement.
Light traveling through a dielectric medium experiences natural scattering through small, random fluctuations in the index of refraction (known as Rayleigh scattering). When highly coherent light propagates through a medium with a large number of scattering sites multiple photons may interfere provided they are scattered in a small volume and are sufficiently in phase (known as coherent Rayleigh scattering or CRS). Over long distances this effect manifests itself as a noise envelope surrounding the natural Rayleigh backscattered signal that reduces the performance of any system attempting to use the signal for calculation.
In practice, high peak laser power and narrow spectral line width are necessary for the observation of CRS. As multi-wavelength Raman and Brillouin based Distributed Temperature Sensors (DTS) attempt to reach longer distances and higher accuracy, laser power has necessarily increased making CRS a commonly observed problem.
Lee, Lee, Won, Park, and Han: “Reduction of Rayleigh Back-Scattering Noise Using RF Tone in RSOA Base Bidirectional Optical Link” [OFC/NFOEC 2008] describes one approach for dealing with this issue by using a reflective semiconductor optical amplifier (RSOA) in optical network units and adding an RF tone into the RSOA.
What is needed is a new approach that simply reduces CRS in DTS systems to enable the use of higher primary laser powers in pursuit of longer range DTS systems.